Automobile body parts are traditionally made of sheet metal or plastics painted with layers of pigmented paints. The painting process of these parts requires elaborate facilities and consequently large expenses. For instance, a large floor space area must be maintained in a clean room environment for the spraying of paint and clear coat and for the baking and curing of such paint and clear coat on the body parts. Moreover, solvent based paints are considered undesirable in recent years because of environmental concerns. As a consequence, the evaporation of such solvents must be strictly controlled. It is, therefore, desirable to eliminate the painting process completely in an automobile assembly plant by using prepainted carrier films. These prepainted carrier films may be applied to prefabricated vehicle body parts in a vacuum thermoforming process.
In the development of a vacuum thermoforming process to assemble a prepainted carrier film to a prefabricated vehicle body part, problems of various nature were encountered. First, in a process where a two dimensional prepainted carrier film is assembled to a three dimensional vehicle body part, it is inevitable that the carrier film must be stretched. The amount of stretch, or the total strain of the carrier film resulted from such stretch varies from a few percent in the case of a flat body panel to as high as one hundred percent in a curved panel with sharp corners. We have found that for most automobile body part applications, the total strain is in the range between 10 to 50 percent.
When a carrier film covered with a color coat is stretched in the vacuum thermoforming process and assembled to a vehicle body part, the color and gloss appearance of the assembled part deteriorates significantly from that of the original unstretched prepainted carrier film. For instance, we have found that in the case of a metallic silver color basecoat, the 20.degree. gloss reading dropped from 87 for an unstretched carrier film to 25 for a carrier film stretched to 50% strain. It is known by those skilled in the art of color matching that such a deterioration in gloss produces a part of unacceptable color appearance.
We have discovered that one possible solution to remedy the deterioration in gloss is by using a clear coat. Clear coats of various chemical nature are available in the market. However, a serious problem was encountered when clear coats are sprayed on a stretched carrier film covered with a color coat. We discovered that since most clear coat formulations contain one or more organic solvents, i.e. ketones, esters, etc., severe stress cracking problems in the color coat layer were observed when these clear coats were sprayed on a stressed color coat layer. We believe that the organic solvents contained in the clear coats initiate crack growth in an already stressed color coat layer which contains micro-cracks. This stress cracking in the color coat layer can cause deterioration in the color appearance to such an extent that the color of the automobile part is no longer acceptable.
It is, therefore, an object of the present invention to assemble prepainted carrier films to automobile body parts in a vacuum thermoforming process.
It is another object of the present invention to assemble prepainted carrier films to automobile body parts in a vacuum thermoforming process in which even though the films and the color coat layer on it are stretched, substantially the same gloss and color appearance are retained after the stretch as prior to the stretch by the use of a clear coat layer.
It is a further object of the present invention to assemble prepainted carrier films to automobile body parts such that all the body parts mounted on an automobile are color matched within an acceptable color tolerance.